In response to the demand for more reliable and higher capacity data storage and retrieval systems, considerable research and development is undertaken on optical disc systems. A primary component of the optical disc system is the optical disc. The optical disc is encoded with information, and serves as the storage component of data storage and retrieval systems. Optical discs are presently available in a variety of formats including, but not limited to, CD-DA (compact disc-digital audio which includes the common music CD), CD-ROM (compact disc-read only memory), CD-WORM (write-once-read-many), CD-R (compact disc-recordable) and DVD (digital versatile disc).
Generally, all optical discs include a substrate, a means for encoding data and a reflective layer. The substrate is typically a polycarbonate disc, with an opening for a spindle being provided in the center of the disc. The encoding of data on an optical disc, in what will be referred to herein as a data layer, can be accomplished in several ways, and in one or more data layers, depending on the desired format. For example, a CD-ROM encodes data directly on the substrate by providing a series of pits and lands on the substrate surface. In contrast, a CD-R utilizes a separate data encoding medium, e.g., a dye or other type of data encoding material that is separate and apart from the substrate, as its data layer.
A reflective layer is required because, as in all optical discs, the data layer is accessed using laser light. More specifically, and as is well known, the reflective layer of the optical discs, as the name implies, functions to reflect laser light onto a detector so the data can be retrieved and subsequently converted into a usable format. To provide the disc with this reflective capability, at least one extremely thin (50-500 nm) reflective metallic (e.g., gold, silver, copper, aluminum or other known reflective metallic alloy) material is deposited (e.g., sputtering, vacuum evaporation) over the data layer of the optical disc. During the data retrieval (reading) process, the laser focuses on and tracks the data located on the optical disc. Because the data layer affects the properties of the laser light, an appropriate detector is able to sense these property differences in the reflected laser light and convert them into a binary (1's and 0's) signal. The binary signal is then further processed to provide the desired output to the user.
The data layer of a DVD, like that of a CD-ROM, comprises a series of pits and lands embossed on the surface of the optical disc substrate. However, to enhance the data capacity of a DVD, two optical discs are typically adhered to one another, with the metallic reflective layers of each disc oriented in back-to-back relation. Currently, there are three methods available for adhering these two optical discs to one another to provide a DVD: hot melt contact adhesives, cationic or PCA UV bonding, and free radical UV bonding. Although cationic bonding provides excellent adhesion, it unfortunately corrodes the reflective metal layer. This corrosion, in turn, causes incomplete or inaccurate data retrieval. In an effort to circumvent this problem, specialized protective coatings are typically applied over the reflective metal layers of such individual DVD discs prior to the application of such adhesives thereto.
In addition to those mentioned previously, there are other problems with existing optical disc technology that must be addressed by optical disc manufacturers. For example, in CD-DA, CD-ROM and CD-R applications, the reflective layer is the outermost layer, and is thus exposed to the environment. Because of this exposure, the reflective layer is subject to physical damage, such as scratches, abrasion and corrosion, which damage the readability of the optical disc. Radiation-curable coatings are thus typically applied directly onto the reflective layers of these discs in an effort to prevent damage thereto.
While there are at present many radiation-curable coating compositions that are promoted for use in connection with optical discs, such compositions must satisfy stringent requirements in order to be suitable for use in optical disc applications. In particular, it has been recognized that an acceptable coating must cure rapidly upon exposure to radiation, be compositionally stable both prior to (storage stability) and after curing (so that the cured coating layer maintains its protective properties for long periods of time), and provide adequate levels of abrasion resistance. In addition, it has further been recognized that the coatings should exhibit a low level of shrinkage, have appropriate levels of hardness, and resist delamination. The coating must also be compatible with the optical disc as a whole, not causing any problems with data retrieval or the reflective metallic material. Further, the coating should not adversely affect the properties of the optical disc, nor be adversely affected itself, even after the optical disc is exposed to elevated temperature and humidity.
Radiation-curable compositions have long been utilized to provide the aforementioned protective coatings for optical discs due, in part, to their ability to provide an acceptable protective coating to those discs under high speed processing conditions. These compositions have typically been produced from an uncured mixture of ingredients, usually reactive acrylates, the compositions being coated onto the optical recording disc and then cured upon exposure to radiation, e.g., electron beam or UV radiation. One known technique of applying a coating layer to an optical disc is by spin coating. The spin coating technique requires that the viscosity of the uncured coating composition be relatively low. For instance, to obtain a coating layer on an optical disc of uniform thickness by spin coating, the coating composition preferably must have a viscosity of, for example, 100 centipoise or less at 25.degree. C. As each disc is coated using this method, the disc is subjected to radiation to cure the composition. The portion of the coating composition applied onto the optical disc that is not retained on the disc and subsequently cured thereon is returned to a holding tank for reuse. It is thus important for a coating composition to retain an acceptably low viscosity despite being repeatedly subjected to low levels of radiation (due to recycling) during an optical disc production run.
Obtaining adequate adhesion of the radiation-curable coating onto the reflective layer of an optical disc, particularly those comprising silver or gold, is also of significant importance. Without such adhesion, and some degree of coating flexibility, delamination of the coating from the optical disc may result. Delamination can cause inaccurate reading of the disc, and can allow moisture, oxygen or environmental pollutants to contact the reflective layer, introducing corrosion and other imperfections onto the optical disc. Many coating compositions are therefore purposefully designed to be very acidic in nature, as highly acidic compositions are known to provide enhanced levels of adhesion to metallic materials.
Further complicating the manufacturing process is that a coating that provides good results in connection with a CD-ROM (having an aluminum reflective layer) may not provide good results when used on a CD-R or DVD (having a silver and/or gold reflective layer), e.g., data retrieval accuracy may vary. In addition, coating compositions used on the exterior of optical discs may not provide sufficient protection to the silver or gold reflective metallic layer of a DVD from cationic adhesive attack. Thus, a manufacturer is forced to order and inventory a number of different radiation-curable compositions for specific applications, e.g., one for a CD-ROM, one for a CD-R with a dye as a data layer, and one for the metallic surface of a DVD.
In addition, the concurrent use of an organic dye as the data layer and silver as the metallic reflective layer on a disc, e.g., a CD-R, has given rise to compatibility problems with existing radiation-coating compositions. For example, existing radiation-curable coating compositions applied onto CD-R discs that include an organic dye and silver reflective layer provide, among other problems, unacceptable levels of jitter, delamination, and clarity.
Thus, there exists a need for a radiation-coating composition that provides acceptable properties on various types of optical discs, particularly on discs that include an organic dye as the data layer and a silver reflective layer. There is a further need for a composition that exhibits, and also provides optical discs with, enhanced performance levels as compared to existing compositions and optical discs.